The present invention relates to fabrication of electronic filter devices for use with quantum computing devices, and more particularly, fabrication of low-pass electronic filter devices for use in cryogenic systems and quantum computing devices.
Current methods in the field of quantum computing perform quantum computations, however, the methods are not capable of meeting practical requirements of a scalable quantum computer. Devices for quantum computers may be based on superconducting Josephson junction devices, or flux qubits. Josephson junction devices are based on the Josephson effect, which is the phenomenon of current flow across two weakly coupled superconductors, separated by a very thin insulating barrier, this arrangement is known as a Josephson junction. The flux qubits include making measurements on superconducting qubit devices (cryogenic systems, high frequency electronics, etc.). A cryogenic system includes a vessel to hold a cryogen, typically helium, in a liquid state with minimal evaporation (boil-off). A shortcoming of these devices includes electronic noise caused by various sources (for example, environment, or equipment). The electronic noise must be fully suppressed, otherwise, the electronic noise can be transferred to the quantum system and lead to premature decoherence of the system. In another aspect of current systems, another problem which occurs is that cryogenic systems (dewars (containers for holding, for example, liquid nitrogen), cryostats, dilution refrigerators, etc.) have limitations in both physical size and cooling power. Thus, no active components (energy dissipating elements) can be located near the superconducting device being tested otherwise thermal dissipation negatively affects the experimental measurements. Known attempts for solving the problem of thermal dissipation include, electrical screen rooms, elimination of ground loops, coaxial/triaxial feedthroughs, low-loss device coupling and terminations, low noise electronics (power supplies, arbitrary waveform generators, pre-amplifiers, oscilloscopes, etc.), and passive and active filtering of signals from room temperature devices. However, these solutions have resulted in shortcomings because most cryostats are limited to just a few micro-watts of cooling power, thus, passive filters (RLC filter networks) near the experimental setup need to be limited in both size and number of elements due to their power dissipation.
Problems regarding quantum computing include determining the ultimate limits of computational speed and efficiency. A quantum computer, if such a device could be manufactured, would be able to solve certain computational problems exponentially faster than even the fastest classical computer. This boost in speed is a result of using quantum bits (qubits), the quantum mechanical analog to the classical bit, which possess the ability to exist not only in the classical states of |0> or |1>, but also in a superposition of those two states. In simplest terms, the qubits can be in both the |0> and |1> states simultaneously. Thus, a collection of qubits, for example, 1000 qubits, could simultaneously store all the logical combinations of 21000 states or about 10300 parallel combinations. If a quantum computing algorithm could be devised to manipulate all thousand qubits at once and if all of these qubits can interact with one another (called entanglement), then all 10300 states can be operated on simultaneously and the results of that algorithm will yield an answer much faster than a classical computers approach of manipulated each of the 10300 states sequentially. However, this is only possible if a superposition can be maintained by all of the qubits.
The principle of superposition is strictly a quantum mechanical phenomenon and can only exist as long as the qubit is isolated from the surrounding environment. Once the qubit interacts with the outside environment, the superposition of the qubit will collapse into one of the two classical states, |1> or |0>. This phenomenon is called decoherence and all qubits, and by extension, all quantum computers, are subject to it. Decoherence time is defined as the time it takes for a qubit to collapse into one of the classical states from its quantum mechanical superposition. Depending on the type of qubit one uses (trapped ion, photons, nuclear spins, superconducting loops, etc.), decoherence times can vary from nanoseconds (109 seconds) to hundreds of microseconds (106 seconds). Thus, if the decoherence time is long enough, qubit manipulations are possible as long as the quantum computation and its associated manipulations can be completed before the qubits decohere. It is apparent that if decoherence times were the only criterion by which to design a quantum computer, then a long-lived qubit would be chosen, however, other criteria may limit the usefulness of a given qubit.
Superconducting qubits based on Josephson junctions may meet criteria for successfully making a quantum computer. Decoherence times for superconducting qubits based on Josephson junctions have been reported to be as long as a few microseconds. Successful manipulations of qubits based on superconducting Josephson junctions require not only the use of cryogenic systems (refrigerators capable of reaching milli-Kelvin temperatures) but also the design and implementation of microwave electronics (Josephson electronics typically operate at microwave frequencies, f>1 GHz). As the dimensions of the qubit, and the wiring that attaches the qubit to the outside world, begin to approach the wavelength of the associated operating frequencies, careful design and implementation of the electronics must be considered in order to not introduce noise and loss into the qubit from the outside world.
As shown in FIG. 1, a potential source of noise can be the electronics used to readout the state of the qubit. A known biasing circuit 98 includes elements 101, 102 and 103, and a superconducting quantum interference device (SQUID) readout circuit including elements 106, 107, and 108, and a qubit 100. Environmental noise, represented by element 105, can be capacitively coupled 104 to a qubit as well. Another potential noise source is the superconducting circuit elements that are used to bias the qubit. In both of these cases, environmental element 105 may be inductively coupled to the qubit circuit. Environmental noise, e.g., thermal current generated in normal metals, stray magnetic fields, etc., can couple into the qubit and be a source of decoherence. While electrical line noises can be filtered out and mitigated using properly designed microwave elements, environmental noise can only be reduced by engineering of the qubit structure, test apparatus and surrounding liquid Helium (He) dewar.
Passive circuit elements, such as resistors, capacitors and inductors, are known to be used as filtering elements in electronics design. The filter type designates the areas of the frequency spectrum which the elements either allow to pass unattenuated or block completely. These filter types are designated as low-pass, high-pass or band-pass filters. A perfect low-pass filter is designed to allows signals to pass unattenuated from DC (0 Hz) up to some prescribed frequency value (fs) after which the electrical signal begins to roll off in strength (called cut-off). Similarly, high-pass filters are designed to allow electrical signals to pass unattenuated from some prescribed frequency (fp) to, theoretically, infinity. Band-pass filters are a combination of low-pass and high-pass filters that either block or allow a specific frequency range. Filters are designed not only based on which frequency portion of the spectrum they allow, but also by how quickly the signal is attenuated (cut-off), how large the attenuation is for the portion of the spectrum that is allowed to pass (pass-band attenuation) for the portion that is blocked (stop-band attenuation), and if there is any allowable ripple in the pass-band or stop-band. These parameters are specified for designing the appropriate passive circuit elements to produce the desired output spectrum or transfer function.
FIG. 2 shows various configurations of circuit elements describe by 4-port networks. Three 4-port low-pass filter networks are shown, 192, 194, 196, the L-filter, T-filter and π-filter, respectively. Each network has an input port 200, 205 and 208, an output port 201, 206 and 209, a common port 202, 207 and 210, respectively, and passive circuit devices elements 203 and 204. The three basic 4-port networks are the L-filter, T-filter and n-filter configurations. Each configuration offers the circuit designer flexibility in creating the various types of filters. The basic elements can be “laddered” together to form filters with a wide range of frequency responses. These laddered circuit elements define different transfer functions for the filter. The transfer function of a filter is defined as the ratio of the filters output signal, usually defined as Y(s), with respect to its input signal, usually defined as X(s), with s being the complex frequency defined as s=+j{acute over (ω)}. Mathematically, the transfer function is defined as,
      H    ⁡          (      s      )        =            Y      ⁡              (        s        )                    X      ⁡              (        s        )            and it will be the ratio of two rational polynomial functions. The order of a particular filter design is defined as the highest power polynomial found in either the numerator or denominator. The structure of the transfer function's rational polynomials determines what family of filters it belongs to with each family having specific characteristics. Examples of filter family types includes, but are not limited to, Butterworth filters, Chebychev filters (both type I and type II), Bessel filters and Elliptic filters. These filter families have specific qualities such as pass band gain and ripple, stop band ripple, cut-off and group delay. If a fast transition between the pass band and stop band is important, elliptic filters have the fastest roll-off of any electronic filter.
Because most passive circuit elements are dissipative (convert energy into heat), utilizing them in a low temperature measurement apparatus can be prohibitive if the thermal energy generated exceeds the cooling power of the low temperature refrigerator, typically less than 50 mW. The filters described above typically require the use of many passive components (>5 components), and thus achieve the necessary filter requirements within the given thermal budget of the system can be impossible.
Low temperature measurements using basic low-pass filters can be made from wires embedded in a matrix of metal powders. These filters can achieve reasonable cut-off levels because the wires will develop a self-inductance that can be enhanced by the addition of metal powders around it. Stray capacitance to ground provides the necessary reactance to achieve a desired cut-off frequency.
FIG. 3 shows a typical metal powder-filled filter. The body of the filter 300, is filled with a metal powder in some non-conducting binder 305. The input and output ports 301 and 302, respectively, are connected to one another by a conductive wire 303, that is then wound around a separate metal powder core 304. The powder core 304 and surrounding powder medium 305 can be of the same or differing compositions depending on the requirements of the filter. Metal powder-filled filters are generally constructed by creating an inductive element and encasing that element in a non-conducting binder that is filled with metal powder. The metal powder can be chosen to be either non-magnetic or ferromagnetic depending on the environmental requirements and the level of attenuation needed. Attenuation in these filters comes mainly from the metal particles absorbing high-frequency radiation and dissipating it through the generation of eddy currents. The fraction of metal powder to binder can be increased until the material mixture becomes conducting, which is the percolation threshold. At this point, there is a low conduction pathway to ground created by the metal powder particles touching one another and the outer case. The inductive element 404 (shown in FIG. 4) can either be a straight length of wire, a coil of wire or a coil of wire that is wrapped around a soft ferromagnetic core to enhance the high frequency reactance of the wire.
The lumped circuit element model for a typical metal powder filter is shown in FIG. 4 which includes capacitors 405. The metal power filter can be designed with cut-off characteristics, however, they suffer from a parasitic capacitance 406 between the input and output ports 301, 302 if proper care is not taken to shield the connectors. This additional parasitic capacitance can provide a pathway for high frequency noise to bypass the inductive element 404, thus leading to poor stop-band performance at gigahertz frequencies (109 Hz).
It would therefore be desirable to provide a method for making compact, highly versatile, superconducting low-pass filters necessary for the electronic readout of superconducting quantum computing devices. It would further be desirable to provide a method for making superconducting low-pass filters out of discrete electronic components which are energy dissipative and not suitable for low temperature (<50 mK) measurement rigs (e.g., dilution refrigerators). It would also be desirable to provide a method for making a superconducting low-pass filter which allows for easy modification of the filter properties (cutoff frequency, transition band slope, etc.).